1. Field of the Invention
The present invention relates to an apparatus for friction stir welding with a pin, which can be driven in a rotating manner about an axis of rotation and has a drive end and a free end, wherein between the drive end and the free end there is provided a cylindrical engaging portion, which extends axially in the direction of the axis of rotation and the circumferential surface of which is intended for engagement with one or more workpieces, with a first shoulder element, which is arranged on the side of the engaging portion that is facing the drive end concentrically around the pin with respect to the axis of rotation and which has a first bearing surface, facing the engaging portion and extending perpendicularly to the axis of rotation, and with a second shoulder element, which is arranged on the side of the engaging portion that is facing the free end concentrically around the pin with respect to the axis of rotation and which has a second bearing surface, facing the engaging portion and extending perpendicularly to the axis of rotation, wherein the second bearing surface extends right up to the engaging portion and wherein the first shoulder element is intended for bearing against the first surface and the second shoulder element is intended for bearing against the second surface, opposite from the first, of one or more workpieces.
2. Discussion of the Prior Art
The principle of friction welding involves the material of one or more workpieces being initially plasticized by friction between the workpiece and a further workpiece or a tool and then solidifying again. In friction stir welding, the two workpieces to be joined, which are generally metallic, are initially placed against one another. This may mean, for example, that two metal plates or sheets are placed surface against surface (lap joint) or else positioned next to one another such that the narrow side faces lie against one another (butt joint). After that, a friction stir welding tool in the region of the contact surface is introduced into the material of at least one of the two workpieces—for example via a bore or from the edge.
The friction stirring process is brought about by the rotation of the tool, the tool rubbing against at least one workpiece and plasticizing the material of at least one of the two workpieces. The friction stir welding tool may optionally be moved at the same time along the interface of the two workpieces. The plasticized material at the interface of the two workpieces subsequently cools down and forms the weld that keeps the two workpieces together.
Numerous apparatuses for friction stir welding are already known from the prior art, such as for example WO 2006/055530. Common to all of these apparatuses is a substantially cylindrical or conical pin with an engaging portion for engaging with at least one workpiece and with a shoulder element for bearing against the surface of the at least one workpiece. The pin and the shoulder element are driven in a rotating manner, the pin penetrating for example between two workpieces that are to be joined and have been placed one against the other so far into the workpieces that the shoulder element bears against the surface of the workpieces.
The friction of the pin and the shoulder element against the workpieces has the effect that the material in the adjacent region of the workpieces plasticizes. With a simultaneous forward movement of the apparatus along the contact surface of the two workpieces, material plasticized by the rotating pin is transported behind the pin, as seen in the direction of movement thereof, where it joins other plasticized material of the two workpieces and forms a weld.
To be able to support the friction stir welding tool on both surfaces and to align the workpieces with one another and press them against one another, an apparatus for friction stir welding with a first and a second shoulder element—known as a bobbin tool—may be used, the first and second shoulder elements bearing against opposing surfaces of the workpiece or workpieces. Such an apparatus is known, for example, from JP 2008/296285 or from EP 1 738 856 B1.
However, the problem with the apparatus for friction stir welding of the bobbin configuration is that, in order to transfer a torque to both shoulder elements and at the same time rub against the region to be plasticized of the at least one workpiece, the pin must absorb very high mechanical and thermal loads. In order not to destroy the apparatus, the achievable speed in the forward movement of the pin is consequently limited by the workpiece, as a result of which the duration of the overall welding process is also increased. However, the limited speed, necessary from mechanical aspects, in the forward movement of the pin leads to excessive process temperatures in the plasticized region of the workpiece and in the region of the pin, which on the one hand may have adverse effects on the quality of the welded joint and on the other hand may damage the pin itself.
Furthermore, when a bobbin tool is used, the process speeds are well below the process speeds of conventional friction stir welding tools. For these reasons, the friction stir welding process using bobbin tools has so far only been of secondary importance.